THREE:For disadvantages there is, on the other hand, a want of uniformity in fittings that prevents their interchange from one part of a line shaft to the othera matter of great importance, as such exchanges are frequently required. A line shaft, when constructed with pieces of varying diameter, is special machinery, adapted to some particular place or duty, and not a standard product that can be regularly manufactured as a staple article by machinists, and thus afforded at a low price. Pulleys, wheels, bearings, and couplings have all to be specially prepared; and in case of a change, or the extension of lines of shafting, cause annoyance, and frequently no little expense, which may all be avoided by having shafts of uniform diameter. The bearings, besides being of varied strength and proportions, are generally in such cases placed at irregular intervals, and the lengths of the different sections of the shaft are sometimes varied to suit their diameter. With line shafts of uniform diameter, everything pertaining to the shaftsuch as hangers, couplings, pulleys, and bearingsis interchangeable; the pulleys, wheels, bearings, or hangers can be placed at pleasure, or changed from one part of the shaft to another, or from [47] one part of the works to another, as occasion may require. The first cost of a line of shafting of uniform diameter, strong enough for a particular duty, is generally less than that of a shaft consisting of sections varying in size. This may at first seem strange, but a computation of the number of supports required, with the expense of special fitting, will in nearly all cases show a saving.The addition of a few questions at the end of each chapter, some of which are not answered in the text, it is thought will assist the main object of the work, which is to promote a habit of logical investigation on the part of learners.
